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A Survey on Power Control Techniques in Femtocell Networks

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A Survey on Power Control Techniques in Femtocell Networks
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  A Survey on Power Control Techniques in Femtocell  Networks Sawsan A. Saad, Mahamod Ismail, and Rosdiadee Nordin  National University of Malaysia, 43600 Bangi, Malaysia Email: {sawsan_3; mahamod; adee}@eng.ukm.my Abstract   —  Rapid increase in mobile data has raised the stakes on developing innovative new technologies and cellular topologies that can meet these demands in an energy efficient manner. One of the most interesting trends that will emerge from this cellular evolution is the femtocell networks. Femtocells are small, inexpensive, low power base stations that are generally consumer deployed, and are expected to significantly improve the coverage and capacity of the indoor users. Femtocell base stations (FBSs) have extensive auto configuration and self-optimization capability to enable simple  plug-and-play deployment. The FBSs perform self-optimization function that continually adjusts the transmit power so the femtocell coverage does not leak into an outdoor area while sufficiently covering the indoor femtocell area. In this paper, different power control techniques in femtocell networks have  been discussed and compared. The focus is on distributed power control techniques due to the decentralized nature of femtocell networks. The conclusion drawn from this review is that the distributed power control techniques using pilot power control schemes are simple and effective in optimizing the coverage of femtocells as well as reducing power consumption of the FBS. Furthermore a novel algorithm is still needed to perform power control optimization in femtocell networks. Index Terms   —  heterogeneous networks, femtocell, power control I.   I  NTRODUCTION  Over the last decades, there is a dramatic increase in the traffic demands. This trend enforces the network service providers to rely on cell splitting or additional carriers to overcome capacity and link budget limitations while guaranteeing uniform user’s quality of service experience. However, this deployment process is complex and iterative. Moreover, site acquisition for macro base stations with towers becomes more difficult in dense urban areas . A more flexible deployment model is needed for operators to improve broadband user experience in ubiquitous and cost effective way [1]. The 3rd Generation partnership Project (3GPP) Long Term Evolution-Advanced (LTE-A) standard has intensively discussed the support of heterogeneous networks (HetNet) [2]. Fig. 1, shows the concept of Manuscript received August 21, 20132013. This work was supported by Universiti Kebangsaan Malaysia under grant DPP-2013-006 and Ministry of Science, Technology and Innovation Malaysia under grant 01-01-02-SF0788. Corresponding author email: sawsan_3@eng.ukm.my. doi:10.12720/jcm.8.12.845-854 HetNet discussed in 3GPP. HetNet is a collection of low  power nodes distributed across a macrocell network to improve the capacity and coverage of the network. There are various types of low power nodes including microcells, picocells, femtocells and relays, which are deployed in various environments including hotspots, homes, enterprise environment and low geometry locations [3]. Table I contains a comparison between some of these nodes. Recent studies on wireless usage show that the major growth in data traffic srcinates indoors [4], where the majority of mobile users suffer from inadequate indoor signal penetration, which lead to poor coverage provided to consumers and they do not enjoy the full data capacity marketed by operators. In the light of the above facts femtocells are now seen as a good solution for providing higher capacity and coverage for in-building network environments [5]. To deploy femtocells in real network environments, there are many problems need to be considered, such as interference avoidance, handover, synchronization, cell selection, and self optimizing networks. Fig. 1. The concept of heterogeneous networks [2]. T ABLE I:   A   C OMPARISON B ETWEEN S OME L OW P OWER  N ODES  Properties Cell Type Microcell Picocell Femtocell Power 30dBm 30dBm 20dBm Coverage range Up to 500m <100m <30m Backhaul X2 interface X2 interface Home  broadband Access mode Open to all users Open to all users Closed subscriber group Deployment Outdoors Indoors or outdoors Indoors Installation By the operator By the operator By the user Cost Expensive Cheap Very cheap 845 ournal of Communications Vol. 8, No. 12, December 2013 ©2013 Engineering and Technology Publishing ; revised October 29,  There are three technical factors that can determine the successful implementation of the femtocell technology: (i) the guaranteed coverage area of the cell, (ii) the auto-configuration and the self-optimization capabilities of the cell, and (iii) the core network signalling caused by the user mobility. The transmit power level of a femtocell base station affects its coverage range and the amount of interference it generates in the network. Although higher femtocells transmit power can provide wider coverage and better signal quality, it can, at the same time, cause tremendous interference to other surrounding users of the adjacent macrocell networks. Properly selecting the femtocell base station transmit power level can help manage the interference from the femtocells to the macro-users, while maintaining femtocells performance. One of the conventional practices is by applying transmitter power control technique. It is widely adopted as it can mitigate the femto-femto (co-tier) as well as femto-macro (cross-tier) interference and increase the network capacity. Power control techniques in cellular networks have  been studied in the literature. References [6,7,8] provide an extensive review on the subject. These papers present the big picture of this research area by classifying the works into its main research sub-areas. The classification in these papers is based on fundamental approaches for  power control, particularly for conventional wireless networks with one tier. However, in femtocell networks, the femto base stations are installed in an ad-hoc manner without proper planning by the network operators or the femtocell owners. This increases the technical challenges for power control due to the decentralized architecture of the femtocell networks and there is uncertainty in terms of coordination between different number of femtocell  base stations in a particular location. Therefore, power control technique approaches in the femtocell environment will be implemented in different ways, which is beyond conventional wireless network planning and optimization techniques. In case of dense deployment of femtocells, a sufficient power control technique is needed, can cope with the ad-hoc nature of femtocells and provide quality of service for both macro and femto users. It should also be kept in mind that the femto base station is a small low powered device and it should be able to handle the complexity of these techniques. Based on these reasons, different considerations should be taken when providing a classification of power control techniques in femtocell networks. The major goal of this paper is to provide a comprehensive survey of power control techniques for femtocell networks. An analytical survey of the current contributions in the literature and comparisons between different power control techniques will be provided. Due to the ad-hoc nature of femtocell networks, the focus is more on distributed power control techniques. In addition, future directions related to the challenges in power control technique will be addressed at the end of this article. This paper is organized as follows: section II provides an overview of femtocell technology and its benefits. The  power control in femtocells and the proposed schemes in the literature are presented in sections III and IV respectively. A comparison between different power control techniques and our future work are presented in section V. Concluding remarks are given in section VI.   II.   F EMTOCELL  N ETWORK A RCHITECTURE AND P ROPERTIES  Femtocell also known as Home enhanced NodeB (HeNB) in 3GPP standardization, is a small size, low  power (<10mW) base station with short service range (<30m) and can support under 10 users simultaneously. Fig. 2, shows the basic femtocell network in 3GPP LTE-A standard [9]. HeNB is considered as a plug-and-play consumer device, which is easily installed by the users. Unlike WiFi, femtocell operates in licensed spectrum owned by the wireless operators and can be deployed with macrocell networks in the same frequency (co-channel deployment) or different frequency (dedicated-channel deployment). Femtocell utilizes the user’s existing broadband internet access (e.g. Digital Subscriber Line (DSL), cable modem, optical fibres, etc.) as a backhaul to communicate with the mobile operator core network [10].  A.    Access Modes One of the key features in any cellular model that includes femtocells is the type of access strategy. There are three types of access control modes for femtocells: (i) closed, (ii) open and (iii) hybrid mode. For the closed access, only closed subscriber group (CSG) is allowed to connect to the femtocell. This mode is preferred by the femtocell owners but it can cause severe cross-tier interference to neighbours that are using macrocell services. On the other hand, the open access mode (OSG) allows all the users to have access to any femtocell network. This could mitigate the cross tier interference but in the cost of increasing the number of handoffs, in addition to increase in signalling on the core network mobility. eNBMME / S-GWMME / S-GWeNBeNB  S 1  S 1  S     1       S    1 X2    X  2 X   2    E-UTRANHeNBHeNBHeNB GW S 1  S 1      S    1  S    1     HeNB S    1     S  1    S 5  MME / S-GW       S   1 X  2   X2   Fig. 2. Femtocell network in LTE-A Systems [9]. 846 ournal of Communications Vol. 8, No. 12, December 2013 ©2013 Engineering and Technology Publishing  In hybrid access mode, only a particular outside users are allowed to access a femtocell. The conditions of access can be defined by each operator separately and entry to any guest or new user can be requested by the owner. The femtocell should have the capability to select the number of outside users to be allowed to access by keeping in view the performance of authorized users in a  particular femtocell [11].  B.    Femtocell Benefits From the technical and business point of view, femtocells offer the following benefits:    Coverage and capacity improvement: Because of their operation on a short transmit-receive distance, a higher signal-to-interference-plus-noise ratio (SINR) could be achieved. These translate into improved reception and higher capacity [12].    High data rates and call quality: The improved coverage enables mobile phones to work at the peak of their capabilities.    Improved macrocell reliability: Femtocell allows operators to offload a significant amount of traffic away from macrocell.    Cost reduction for mobile operators: Femtocell minimizes the capital and operational expenditure by reducing the additional time for installation and operation cost.    Compelling new femtozone services: Using the specific knowledge of the user’s location femtocell can offer extra benefits such as control of devices around the home.    Simple deployment: Femtocells are designed to be simple to install, configure and operate (zero touch installation) [13]. III.   P OWER C ONTROL IN F EMTOCELLS  The transmit power of HeNB consists of: (i) pilot  power (responsible for cell selection and channel estimation) and, (ii) traffic power (includes signalling  power and data power). Pilot power determines the coverage of the cell: large pilot power results in a large cell coverage and small pilot power may lead to insufficient coverage. Moreover the larger the pilot power the less the power left for traffic, which result in minimizing the throughput of the femtocell. Furthermore, large pilot power may introduce high outage probability to neighbouring non-CSG users due to interference [14]. HeNBs have extensive self-organization capabilities to enable simple plug-and-play deployment and are designed to automatically integrate themselves into an existing macrocell network. These self-capabilities are implemented using an algorithm that automatically changes certain network configuration parameters (radiated power, channels, neighbour list, and handover  parameters) in response to any change in the environment it is operating in. In order to successfully execute the  process of self-organization, there are three main functions need to be performed: (i) Self-configuration in  pre-operational state, (ii) Self-optimization in an operational state, and (iii) Self-healing in case of failure of a network element. One example of self-organizing capabilities in femtocell deployments is power optimization. In this way a self-configuration function can transmit power based on the measurement of interference from neighbouring base stations in a manner that achieves roughly constant cell coverage. The HeNB then performs a self-optimization function that continually adjusts the transmit power, so that the femtocell coverage does not leak into an outdoor area while sufficiently provides indoor coverage [15]. In general power control is adopted for at least one of the following reasons: (i) to mitigate the interference in order to increase capacity of the network, (ii) to conserve energy in order to prolong battery live, and (iii) to adapt to channel variations in order to support Quality of Service (QoS) [8]. Power control algorithms are advantageous in that the macro-eNB (MeNB) and HeNB can use the entire  bandwidth with interference coordination. The dynamic  power setting can be performed either in a proactive or interactive manner each of which again can be performed either in open loop power setting (OLPS) mode, where the HeNB adjust its transmission power based on its measurement results or a predefined system parameters, or closed loop power setting (CLPS) mode in which the  power adjustment is done by the HeNB based on the coordination with MeNB [16]. IV.   C LASSIFICATION OF P OWER C ONTROL T ECHNIQUES  Different power control techniques in femtocell networks have been proposed in the literature. These techniques could be classified according to different criteria such as non- assistance-based vs. assistance-based, centralized vs. distributed techniques. We hereafter  present a brief overview of the state of the art on power control techniques. Fig. 3, illustrates the classification of  power control techniques in femtocell networks that we are going to analyze in the following sections.  A. Non-Assistance-Based Methods In this category HeNB sets its transmission power according to itself measurement reports or predetermined system parameters. 1)    Fixed HeNB power setting   In this scheme HeNB transmission power is fixed. This means that the HeNB would not consider any surrounding information. In this case, when a macro user (MUE) is close to a HeNB which is deployed at the edge of MeNB cell, the interference from the HeNB to the MUE would be high since the desired signal level from MeNB to MUE would be smaller than the interference signal from HeNB to MUE. This would lead to a need for adjustable HeNB power setting scheme based on surrounding information [17]. 847 ournal of Communications Vol. 8, No. 12, December 2013 ©2013 Engineering and Technology Publishing  Power control techniques in femtocell networks Non-assistance-based techniquesAssistance-based techniquesCentralized techniquesDistributed techniquesHeNB measurement reportCoordination with MeNBLocation dependentInterference avoidancePilot power controlPredefined system parametersReports from MUE/HUE   Fig. 3. Classification of power control techniques in femtocell network. 2)   Strongest MeNB received power at HeNB  In [17] the power setting is based on the strongest MeNB received power at HeNB. In this technique, HeNB adjusts its transmission power according to the following formula:     max min max min . , , [dBm] tx m  P P P P        (1) where  , P  max  and  P  min  are the maximum and minimum transmit power of the HeNB, respectively,  P  m  denote the received power from the strongest co-channel MeNB on HeNB. α  is a linear scalar that allows altering the slope of  power control mapping curve, and β  is a parameter expressed in dB that can be used for altering the exact range of  P  m covered by dynamic range of power control. However the measurement of MeNB received power might be insufficient in some operation scenarios, especially when HeNB and HUEs are located in different rooms or floors as shown in Fig. 4. Therefore without the measurement reports from MeNB/MUE/HUE, this technique may determine an inaccurate value for the HeNB power setting. When HeNB measures a relatively higher RSRP from MeNB as shown in Fig. 4 (a), it may set a relatively higher transmit power. This may increase the interference level to the MUEs. On the other hand, when HeNB measures a relatively lower RSRP from MeNB as depicted in Fig. 4 (b), it may set a relatively lower transmit power. Although the interference to MUEs is decreased, the HeNB coverage is significantly reduced. Therefore, this mechanism could be adopted when HeNB is turned on or when HeNB cannot receive measurement reports from HUE/MUE.  B. Assistance-Based Methods In this category HeNB sets its transmission power  based on measurement reports from MUE/HUE or the coordination with MeNB. 1)    Pathloss between HeNB and MUE The transmission power of the HeNB is set as:   max min , , [dBm] tx m offset   P median P P P P     (2) where power offset is defined by:   int max min , , offset er pathloss offset offset   P median P P P       (3) with  P  inter_pathloss   denoting a power offset value that captures the indoor pathloss and penetration loss between HeNB and the nearest MUE, and  P  offset_max ,  P  offset_min   are the maximum and minimum values of power offset, respectively [18]. This method can provide better interference mitigation for MUE and maintain good HeNB coverage for HUE. However the signalling between the HeNB and MUE or  between MeNB and HeNB is required. If HeNB cannot successfully receive the MUE measurement reports, this method cannot work properly. 2)   Objective SINR of HUE HeNB sets its transmission power based on HUE measurement and the objective SINR of HUE, using the following equation.     min _ max max ,min , [dBm]  HeNB estimation HUE received   P P PL P P     (4) HeNBMUE P  r      R   S   R   P  o   f  a   M  e   N   B  RSRP FUE   I n  t e r  f e r e n c e   (a) Small penetration loss (b) Large penetration loss Fig. 4. Scenarios where HeNB transmit power is not appropriately set. where the  PL estimation   is the pathloss estimated between HeNB and the HUE, with 848 ournal of Communications Vol. 8, No. 12, December 2013 ©2013 Engineering and Technology Publishing    0  1010 _ 10 10log 10 10 [dBm]  N  I  HUE received   P x     (5) where,  I   is the interference detected by the served UE,  N  0   is the background noise value,  x  is the target SINR for the HUE [19]. The purpose of this technique is to reduce the interference to MUE by restricting received power of HUE to a desired level. Since HUE is connected to HeNB, HUE can report the measurement information to its serving HeNB, which can enhance the accuracy of power setting. 3)   Objective SINR of MUE This method is based on SINR sensing by MUE, and the HeNB transmit power is given by:     max min max min . , , [dBm] tx SINR  P P P P        (6) where, the  P  SINR  is defined as the SINR between the MUE and the nearest HeNB [20]. The aim of this method is to guarantee a minimum SINR at the MUEs to protect the reception of control channel of MUEs. Table II represents a comparative summary of assistance-based vs. non-assistance-based power control techniques for femtocell networks. T ABLE II:   S UMMARY OF A SSISTANCE -B ASED VS .    N ON -A SSISTANCE -B ASED P OWER C ONTROL T ECHNIQUES  N ODES  Scheme Scheme Classification Scheme  principle Cooperation among femto and macro Access mode Advantage Disadvantage Fixed HeNB  power setting [17]   Assistance- based Predetermined system  parameters.  Not required   Closed, open, and hybrid   (i) Only 1.0% of FUEs could  be in outage. (ii) High FUE throughput (56.6Mbps). (iii)Simple and easy to implement. 18% of MUEs could be in outage. Strongest MeNB received  power at HeNB [17]. Assistance- based HeNB Measurement reports  Not required Closed, open, and hybrid (i) MUEs outage is reduced to 8.7%. (ii) Suitable for scenarios where the reports from MeNB/MUE/HUE are not available. (i) FUE throughput decreased to 46.7Mbps. (ii) Measurements might be insufficient in some operation scenarios. Pathloss  between HeNB and MUE [18].  Non-assistance- based HeNB and MUE measurement reports Required Open and hybrid (i) Outage reduction up to 7.4% for MUEs. (ii) Maintaining good HeNB coverage. HeNB should communicate with MeNB or MUE to get reports from MUE and that would have a great impact on RAN specification. Objective SINR of HUE [19].  Non-assistance- based HUE measurements and objective SINR of HUE  Not required Closed, open, and hybrid. Improve the cell- edge HUEs spectral efficiency by 11.9% compared to no PC scenario. This method cannot work  before HeNB receives the signalling from HUE. Objective SINR of MUE [20].  Non-assistance- based SINR sensing of MUE Required Open and hybrid The outage of MUEs is minimized to 6.0%. (i) FUE throughput is reduced to 37.2Mbps compared to FPS. (ii) MUE need to report SINR to HeNB however there is no direct connection between HeNB and MUE . HeNBHUEHUEHUEHUEHUEHUEHUEHeNBHeNBHeNBHeNBHeNBHeNB Macro BS MUE      A  g  g  r  e  g  a   t  e   d   I  n   t  e  r   f  e  r  e  n  c  e   K=2/4=0.5Pr* =Pr/k =2Pr (k=active femtocell ratio) Macro BS    P r P    r    K=2/3=0.67Pr* =Pr/k =1.5Pr (k=active femtocell ratio) OFFPr*ONPr*OFFONOFFONPr*ONPr* ID   Activity   1   ON   2   OFF   3   OFF   4   ON   ID   Activity   1   ON   2   OFF   3   ON   MUE HeNBHUE   (a) Centralized sensing (b) Distributed sensing Fig. 5. Sensing-based opportunistic power control [21]. 849 ournal of Communications Vol. 8, No. 12, December 2013 ©2013 Engineering and Technology Publishing
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